1. Field of the Invention
The present invention relates to a composite electronic module in which a non-reciprocal circuit element is mounted on a substrate.
2. Description of the Related Art
Hitherto, a non-reciprocal circuit element, e.g., an isolator or a circulator, has been used in a composite electronic module, e.g., a power amplification module in a transmitting circuit section of a communication terminal, such as a cellular phone and a wireless LAN device, by utilizing a characteristic of the non-reciprocal circuit element to transmit a signal in a predetermined particular direction. As illustrated in FIGS. 7A and 7B, for example, a non-reciprocal circuit element 500 mounted on a substrate and forming the above-mentioned type of composite electronic module is formed in such a state that a ferrite 505 is sandwiched between a pair of permanent magnets 501 each having a rectangular parallelepiped shape and including center electrodes 503 and 504, which are formed on a principal surface 502 thereof and which are electrically insulated from each other (see, for example, Japanese Unexamined Patent Application Publication No. 2006-311455, Japanese Unexamined Patent Application Publication No. 2007-208943, and Japanese Unexamined Patent Application Publication No. 2009-49879).
Furthermore, the ferrite 505 has a rectangular parallelepiped shape and includes relay electrodes 506 that are formed in an upper end surface and a lower end surface thereof for electrical connection to the center electrodes 503 and 504 formed in the permanent magnets 501. By forming the non-reciprocal circuit element 500 as described above, manufacturing can be facilitated and an element size can be reduced in comparison with a related-art non-reciprocal circuit element having a structure in which a ferrite including copper wires wound around the ferrite is disposed as a center electrode between a pair of permanent magnets. In order to suppress the influence of a magnetic field formed by the permanent magnets 501 upon other electronic components mounted on a mother board, the non-reciprocal circuit element 500 is mounted on the substrate together with a metal yoke, which functions as an electromagnetic shield, thereby forming various types of composite electronic modules. It is to be noted that FIGS. 7A and 7B illustrate one example of the related-art non-reciprocal circuit element 500; specifically, FIG. 7A is an exploded perspective view of the non-reciprocal circuit element 500, and FIG. 7B is a perspective view of the non-reciprocal circuit element 500.
Recently, various types of composite electronic modules incorporated in communication terminals have been demanded to have smaller sizes and lower heights with further reduction in size and height of the communication terminals. In order to reduce the size and the height of the composite electronic module in view of such a demand, it is conceivable to form the composite electronic module by mounting the non-reciprocal circuit element 500 on the substrate with omission of the yoke. With omission of the yoke, the smaller size and the lower height of the composite electronic module can be achieved because it is not necessary to secure a space for mounting the yoke on the substrate.
In that case, however, since the yoke functioning as the electromagnetic shield is not mounted on the substrate, the number of lines of magnetic force generated by the permanent magnets 501 and leaking to the outside of the substrate forming the composite electronic module increases. Accordingly, the influence of the magnetic field formed by the permanent magnets 501 increases outside the substrate of the composite electronic module. For example, when the composite electronic module in which the yoke is omitted as described above is mounted to, e.g., a mother board, there is a possibility that other electronic components mounted in the surrounding area of the composite electronic module on the mother board may be moved to shift from the desired position by the magnetic force of the permanent magnets 501. For that reason, a countermeasure for preventing such a position shift is required.